When two different frequencies interfere with each other they give rise to a third frequency which is equal to the difference between them i.e.
frequency A Hz and frequency B Hz will produce frequency (A - B) Hz.
This is called a beat frequency. Beat frequencies are used to entrain the brain to produce frequencies of the same rate.
With sound you can have two different frequencies on respective headphones and the interaction will generate a beat frequency that will be perceived by the brain as coming from inside it
and thus it will be easier for the brain to entrain to this specific frequency.
